The present invention relates to a power supply device having a charging power supply such as an alternator or a battery and a plurality of capacitors for supplying electric energy, which has been charged by the charging power supply, to an electric load.
Lead storage batteries are mounted on motor vehicles as a power supply for an engine starter and other electric loads such as accessories.
When the engine is to be started, electric energy stored in the load storage battery is supplied to energize the starter motor. A pinion gear of the starter motor is brought into mesh with a ring gear mounted on the crankshaft of the engine, and rotated to rotate the crankshaft, thereby starting the engine.
An electric current which is supplied from the battery to the starter motor when starting the engine is very high, e.g., 100 A or more, though it is supplied in a short period of time. The capacity of a battery to be installed on a motor vehicle is determined primarily in view of its ability to start the engine. The large electric power which is consumed to start the engine is supplemented when the battery is charged by electric power generated by an alternator mounted on the motor vehicle and driven by the engine while the motor vehicle is running.
Batteries mounted on motor vehicles are known lead batteries as secondary batteries, and they are charged and discharged through a chemical reaction between electrodes and an electrolytic solution. Such a battery can discharge a large current within a short period of time. The battery is charged with a current of 10 A or less which is supplied over a long period of time and through a gradual chemical reaction. Therefore, if a much larger current is supplied to charge the battery, the battery would be excessively heated and the electrodes might be deformed and damaged.
Motor vehicles which are mainly used by commuters run over short distances, and motor vehicles used as delivery cars are repeatedly stopped and started highly frequently. Since these motor vehicles require the engines to be started frequently and are continuously driven over short periods of time, the batteries mounted on these motor vehicles cannot be charged sufficiently enough to make up for the electric power consumed when the engines are started. Accordingly, the batteries tend to be used up, or run down, failing to start the engines.
To solve the above problems, the applicant has proposed a motor vehicle power supply device which has a large-capacitance capacitor that is charged by a battery mounted on the motor vehicle and that discharges stored electric energy to actuate the engine starter, thereby starting the engine (see Japanese Patent Application No. 63(1988)-329,846, U.S. patent application Ser. No. 454,267 and EPC Patent Application No. 89313559.0).
With the proposed motor vehicle power supply devices, the engine starter is energized by the electric energy stored in the large-capacitance capacitor. The large-capacitance capacitor can supply an intensive current. Therefore, even if the voltage of the battery is somewhat lowered, the engine may be started by the electric energy stored in the capacitor. To charge the capacitor after its stored electric energy has been discharged, a large initial current is required, and hence the alternator and the battery and the large-capacitance capacitor have to be connected by thick harnesses which can withstand large currents.
FIG. 3 of the accompanying drawings schematically shows a circuit arrangement in which a plurality (e.g., three) of capacitors are employed to provide a large capacitance. The capacitors are charged by an alternator, and a starter is connected as an electric load to the capacitors. The capacitors, which are connected parallel to each other, and the alternator are coupled to each other by thick harnesses. During an initial stage of the process of charging the capacitors, a large current flows through the harnesses into the capacitors. The large current tends to shorten the service life of the alternator since the alternator is overloaded.